In the production of steel or other metal strip, it is a common practice to provide a continuously operable strip-processing line containing a cold after-rolling stand having a pair of rolls which engage the strip between them. After-rolling stands of this type are provided for various purposes and in various strip-processing lines. For example, they are known in zinc-coating, aluminizing and annealing lines to alter the crystallography of a strip or a coating thereon, to calibrate (i.e., adjust the thickness) the strip, or to effect some other type of surface conditioning.
The after-rolling stand is generally provided between a driving bridle and a retarding bridle, possibly in combination with a stretch-bending roller assembly constituting a strip straightener. Each such bridle, as is well known, may comprise at least one pair or rollers with the strip passing around the major part of the circumference of one roller and then around the major part of the circumference of the other roller so that the two rollers of the pair apply substantial frictional force to the strip.
The advantage of a strip-process line of the aforedescribed type is that a separate rolling process need not be carried out with storage of the strip after a previous rolling and before the after-rolling, special transport means for manipulating or handling the strip materials being thereby obviated.
With increasing frequency, such strip-processing lines are provided, at their discharge ends with table shears at which lengths of strip are severed. The after-rolling must be carried out previously in such cases. The after-rolling has as its principal purpose the modification of the surface structure of the band to achieve a desirable grain or crystal character or to modify the roughness properties.
When metal coatings are applied to the steel, the cold after-rolling may be used to break up an undesirable crystal structure or increase the fineness of the grain of the coating layer as noted. Cold after-rolling stands have not, as a rule, been operated at their full capacity (speed) in conventional strip-processing lines since an absolute agreement between the strip-velocity controllers for the remainder of the processing line and the cold after-rolling stand cannot be achieved in practice. As a result, the cold after-rolling stand is generally operated as an entrained unit, i.e., the rolls are dragged around by entrainment with the strip which is otherwise displaced, e.g., at the downstream bridle or thereafter. In this case, the rolls of the after-rolling stand do not have respective drive motors for the upper and lower rolls.
Customarily a continuously processed strip comprises lengths of the metal strip which are welded together in end-to-end relationship, the weld seams passing periodically or, more accurately, intermittently between the upper and lower rolls of the cold after-rolling stand. To prevent these weld seams from damaging the roll surfaces, it is common practice to provide means for spreading the rolls apart for the passage of each weld seam.
While this prevents damage to the rolls by the weld seams, it introduces another problem since at least the upper roll ceases to contact the strip and the peripheral speed of this upper roll may lag behind the surface speed of the strip (i.e., its linear velocity) so that, when the two rolls are again brought into forceful engagement with the strip for the after-rolling thereof, the surface of the upper roll upon contact with the strip may be at a lower velocity.
Since the strip moving at high speed and coming into contact with a lower speed rolling surface may be marred, it has already been proposed to accelerate a lagging roll of a cold after-rolling stand to a peripheral speed which is equal to the linear velocity or surface speed of the strip. Such systems have been applied to drag rolling stands of the type previously mentioned and are only effective when the rolls are spread away from the strip and must be controlled by hand.
When the resulting manual setting of the peripheral speeds of the upper and lower rolls is not precise, upon contact of the rolling surfaces with the strip, the resulting slippage will give rise to surface defects and even, in the case of thin sheet metal strip, to tearing thereof.
Another disadvantage resides in that a high rolling rate for the cold after-rolling stand having dragged or entrained upper and lower rolls, is that increasing rolling rates require increasing strip tension especially with thin strip, to which high rolling pressures are applied, the strip cross-section is not capable of withstanding the tension forces which must be applied for high rolling rates and the strips tear.
Difficulties are also encountered when the after-rolling stand is combined with a stretch-bending straightening device, i.e., a roller leveler, since the high tension forces may give rise to excessive stretching at the roller leveler. This cannot be reduced by reducing the degree to which the stretch-bending rollers project into the horizontal plane of the strip since this reduces the effectiveness of the leveler.
Moreover, with reduced bending stresses in the roller leveler, the internal stresses in the strip cannot be fully equalized and the desired planar anisotropy, shape-change characteristics, especially for low-carbon steel strip, cannot be obtained. Finally, in this connection, the desired degree of tension cannot be maintained at the stretch bending or roller leveler.
Accordingly it has been proposed to provide, separate from the cold after-rolling stand, special breaking and tensioning bridles and even to provide the roller leveler with separate strip-tensioning bridles so that the desired degree of tension can be maintained for a given strip-transport rate, the desired degree of penetration of the leveling rollers etc.
These expedients have been found to be very costly and, in some cases, unreliable.